Catalysts are employed in many chemical industries to facilitate chemical reactions which would otherwise require unduly stringent reaction conditions. Many catalysts are based on alumina, used either alone (such as in Claus conversion of hydrogen sulfide, alcohol dehydration, and olefin isomerization), as a support (such as the pt/pd catalysts used in automobile exhaust systems and in gasoline reforming), and as an interactive support (particularly in hydrorefining catalysts for petroleum distillates and residual oils).
Typical hydrorefining catalysts comprise a Group VIII metal (typically nickel or cobalt) and a Group VIB metal (typically molybdenum or tungsten) supported on an alumina base. The preparation of such a catalyst may be exemplified by U.S. Pat. No. 4,113,661 to Tamm, the disclosure of which is incorporated herein by reference. The alumina present in such catalysts is typically .gamma.-alumina, though some catalysts prepared by higher-temperature calcination contain .delta.- or .theta.-alumina. The catalysts are typically particulate, with sizes ranging from 0.8 mm diameter cylinders to 5-10 mm diameter cylinders, and more complex shapes such as ovals, trilobes, rings, etc. being common.
In the hydrorefining process, the catalyst typically picks up metals such as nickel and vanadium from the hydrocarbonaceous feedstock, and is frequently contaminated by iron which has made its way into the feedstock from, e.g., the walls of storage vessels, tubing, etc. as they corrode. Depending on the catalyst, hydrorefining conditions, etc., the nickel and vanadium tend to deposit throughout the catalyst by lining the pores with which it is permeated, but the iron tends to stay on the catalyst surface. These metals tend to be deposited as sulfides due to the presence of sulfur compounds in the feedstock. Hydrocarbonaceous and carbonaceous materials, such as coke, also tend to deposit in the pores, and on the surface of the catalyst. The catalyst is said to be "spent" when its catalytic ability for hydrorefining decreases below an acceptable level.
Other alumina-base catalysts are used for other processes, particularly in the petroleum industry. Cracking catalysts may to some extent be regenerated by roasting them in air to burn off deposits, since metals deposition is less significant, but eventually they become economically incapable of regeneration.
At this point, the problem of disposal of the spent catalyst arises. While the hydrocarbonaceous and carbonaceous deposits may be largely removed from the catalyst particles by roasting, oxidizing the deposits to water and carbon oxides, the metals remain. Some of the metals are sufficiently valuable to warrant recovery; others are not.
Previously, spent catalyst would simply be dumped into a landfill or similarly disposed of; but, as the price of catalytic metals has risen, it has become economically attractive to recover all or at least a portion of those metals, such as cobalt and molybdenum. U.S. Pat. Nos. 3,567,433 to Gutnikov and 4,409,190 to Van Leirsburg, the disclosures of both of which are incorporated herein by reference, exemplify processes for the recovery of metals from spent catalysts by air-roasting the catalyst followed by one or more leaching steps.
Although these processes can remove a substantial portion of the metals present, they do not remove all, and a catalyst residue is left which consists essentially of the base and a few tenths of a percent to a few percent of the catalytic or feedstock metals. From the point of view of metals recovery, it is not economic to treat this residue further, and the residue is usually disposed of in a landfill.
However, because there remain on the catalyst base some metals which are potentially capable of leaching into the environment from a landfill or dump site, the residue may be classified as a hazardous waste. In that case, disposal at a hazardous waste site at substantially greater expense may be required.
It would thus be economically desirable to be able to completely recycle the spent catalyst including the base; but the forms of alumina found in catalyst bases are typically not readily soluble in basic solutions (as, e.g., in the Bayer process), and, although they are to some extent soluble in acid solutions, this offers no particular advantage in disposal or recycling.